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Marine Photobank's mission is to advance ocean conservation by providing free, high-quality marine pictures to media and noncommercial outlets. For this photo contest, Marine Photobank was looking for powerful images that "illuminate the many threats facing our ocean." (The National Geographic Society, which owns National Geographic News, donated prizes for the contest winners.)

The shark in the photograph was accidentally caught in a longline, a fishing method that often results in harvests of unwanted marine species. (Take an ocean-issues quiz.)

During an expedition in July 2011, photographer Terry Goss saw many other sharks with embedded hooks or with cuts and marks from hooks, lines, and nets.

"One shark had several plastic box ties wrapped tightly around his body, and another looked to be recently recovering from having his jaw ripped open—practically to the gill slits," Goss said in a statement.

What does Goss want people to take away from his winning picture? "Sharks rule," and "it's such a shame he's injured."

Frozen tuna are transferred from a fishing vessel to a carrier vessel in the Indian Ocean in a picture that won an honorable mention in the 2011 ocean photo contest.

Many species of tuna are being overfished, particularly giant bluefin tuna, which can grow to 12 feet (3.7 meters) in length, weigh 1,500 pounds (680 kilograms), and live for 30 years.

Giant bluefin once migrated by the millions throughout the Atlantic Basin and the Mediterranean Sea, but the fish are now experiencing significant declines. By the mid-1990s, stocks of southern bluefin tuna had been fished to between 6 and 12 percent of their original numbers in the South Pacific and Indian Oceans.

The picture, which won an honorable mention as the "Most Hopeful" photograph in the 2011 contest, "demonstrates the infectious power of community-based beach cleanups and how local children can play a role in environmental stewardship," according to the Marine Photobank website.

A crab gets tangled in a gill net that had settled on a coral reef in Hawaii.

The picture, which won an honorable mention in the 2011 photo contest, "shows the danger posed by abandoned gill nets to coral reef ecosystems, and how the nets continue to fish long after the fishermen have left the location," according to the Marine Photobank website.